The spring transition of thermal stratification on a mid-latitude continental shelf

The spring transition from the wintertime horizontal thermal stratification to the summertime vertically stratified state on a wide, sloping mid-latitude continental shelf is investigated. The Sigma Z-Level Model (SZM), a new three-dimensional primitive equation numerical ocean model employing a hybrid sigma coordinate and z-level coordinate in the vertical, simulates the continental shelf region. A simple analytical model to describe the heating rate of a well-mixed water column is introduced and applied to the problem to illustrate the importance of bottom slope in determining the evolution of the horizontal thermal stratification. The applicability of this simple model to continental shelf dynamics forced by realistic surface fluxes is verified by running a "pseudo" two-dimensional (x-z) form of the SZM with daily heat and wind stress records from the West Florida shelf. A term-by-term evaluation of the depth-averaged time-integrated temperature equation quantifies the significance of the physical processes that can alter the horizontal temperature gradient. The simple model used to describe the heating rate of a homogeneous water column appears as the dominant balance in the integrated temperature equation, and agrees to the SZM model results from the simulation with realistic forcing to within 31% error over the shelf. Horizontal and vertical advection during periods of downwelling result in faster heating over the inner shelf than predicted by the simple model. It is shown that two processes describe the spring transition. The first is the erosion of the wintertime horizontal thermal gradient and occurs on a time scale of about two to three months, dependent upon the heat input, when the water column is being heated and mixed periodically. The second process is the formation of the seasonal thermocline following the last strong mixing event. This occurs on a time scale of less than a week under a stabilizing heat flux and light winds completing the spring transition from horizontal to vertical thermal stratification.